1. Field of the Invention
The invention is related to metal extrusion devices and in particular to a carbide mandrel for use in the extrusion of hollow thin walled articles.
2. Description of the Prior Art
Conventionally, the devices for the extrusion of hollow metal articles have steel mandrels directing the flow of the extruded metals to produce an internal cavity or a plurality of cavities. These steel mandrels as taught by Yamaguchi et al in U.S. Pat. No. 3,808,860; Lugosi, in U.S. Pat. No. 4,085,606; and Cleve et al in U.S. Pat. No. 4,779,440, are supported at ends opposite the extrusion die by a radial flange which engages a support member. These mandrels appear to work well for larger extruded articles, but their life is limited because at the extrusion temperature the mandrels anneal out and become soft after extended use.
This problem is exaggerated for devices designed for micro extrusion of thin walled hollow articles having wall and web thicknesses ranging from 0.020 to 0.012 inches such as would be used in heat exchangers for refrigerators or air conditioning units. It has been found that the frictional heat and heat resulting from a change of phase generated during the extrusion process of these thin walled hollow articles rapidly anneal out the steel mandrel which leads to their premature deterioration or bending.
The use of sintered carbide dies having significantly high softening temperatures is well known in the art, however, the use of sintered carbide mandrels has been limited to relatively large round mandrels. In a circular form, the sintered carbide has a fairly uniform coefficient of thermal expansion. Conversely, however, rectangular mandrels made from sintered carbide have nonuniform coefficients of thermal expansion and will readily fracture in areas of concentrated stresses during the extrusion process, particularly in the region of the radial flange as taught by the prior art. The prior art does not teach or infer any method or structural arrangement which overcomes the problems resulting in the nonuniform coefficient of thermal expansion of rectangular carbide mandrels which would permit the use of carbide mandrels in extrusion devices for making hollow thin walled articles having wall thicknesses less than 0.025 inches.